Add magefiles directory support (#405)

* Use magefolder if no directory set and it exists.

If no directory was passed by the user as an explicit option and there is a folder named "magefolder" use that.

Workdir is kept as it is likely still "."

* Remove the default . for -d flag

Also correct os.Stat error checking to expect no error

* Add tests and test data for magefolder

* Rename magefolder and accept untagged files

Magefolder was renamed to magefiles
We now accept files that are not tagged too when using a magefiles directory

* Assume tagging when mix tagging is present

When using magefiles directory, if there are mixed tagging files assume tagging is used for mage files

* Update error format to %v

We support building for older go versions so error formatting should use %v

* sort outputs

* Accept mixed tagging in magefiles folder

When mixed tagging is found within a magefiles folder, opt to use all files

* little tweak to only do go list once when using magefiles directory

* Add magefiles directory information to the website

* Add a preference for mage files over directories

Add a temporary preference for mage files over magefiles directories and warn users this is a temporary functionality leading to a change where directory will be preferred.

Co-authored-by: Nate Finch <natefinch@github.com>